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UCLA Electronic Theses and Dissertations UCLA UCLA Electronic Theses and Dissertations Title Classification of a Family of Free Bogoliubov Actions of R on L(F∞) up to Cocycle Conjugacy Permalink https://escholarship.org/uc/item/6pd8j0d0 Author Keneda, Joshua Tyler Publication Date 2019 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA Los Angeles Classification of a Family of Free Bogoliubov Actions of R on L(F1) up to Cocycle Conjugacy A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Mathematics by Joshua Keneda 2019 c Copyright by Joshua Keneda 2019 ABSTRACT OF THE DISSERTATION Classification of a Family of Free Bogoliubov Actions of R on L(F1) up to Cocycle Conjugacy by Joshua Keneda Doctor of Philosophy in Mathematics University of California, Los Angeles, 2019 Professor Dimitri Shlyakhtenko, Chair Much of the research into operator algebras concerns the classification problem for von Neumann algebras, where one hopes to find useful invariants to categorize von Neumann algebras up to isomorphism. Murray and von Neumann initiated the study of these objects and reduced the classification problem to that of classifying the so-called factors. And while Type I factors are fully understood, the Type II and Type III cases are still active areas of research. Relatedly, one can attempt to classify group actions on von Neumann algebras up to a suitable notion of equivalence, for example unitary or cocycle conjugacy. The aim of this dissertation is to classify a family of free Bogoliubov actions of R on the Type II1 free group factor L(F1) up to cocycle conjugacy. We consider a certain collection C of measure classes on R, corresponding to the spectral measure classes of the infinitesimal generators for orthogonal representations α : R !O(H), with H separable and infinite- α dimensional. Such representations give rise to a free Bogoliubov action σ of R on L(F1) and an associated von Neumann algebra: the crossed product L(F1) oσα R. Note that the cocycle conjugacy of two actions gives an isomorphism between their crossed product von Neumann algebras. We show that our family of free Bogoliubov actions are completely classified up to cocycle conjugacy by their associated spectral measure class [α] 2 C. Our main technical tool for this result mirrors a recent result of Houdayer, Shlyakhtenko, and Vaes and relates the equality of the spectral measure classes [α]; [β] to the embeddability ii (in the sense of Popa's intertwining-by-bimodules) of the group algebra Lα(R) into Lβ(R) inside of their shared crossed product M ' L(F1) oσα R ' L(F1) oσβ R. By restricting to representations α of R which act trivially on a large subspace of H, we force Lα(R) to have large (non-amenable) relative commutant in L(F1) oσα R. This allows us to use solidity arguments and a rigidity result of Houdayer and Ueda to \trap" Lα(R) inside of Lβ(R) within the crossed product (i.e. Lα(R) M Lβ(R)). Applying the technical tool of the previous paragraph, we obtain that [α] = [β], which establishes the desired cocycle conjugacy invariant. iii The dissertation of Joshua Keneda is approved. Rowan Killip Tim Austin Sorin Popa Dimitri Shlyakhtenko, Committee Chair University of California, Los Angeles 2019 iv To my family, whose love and kindness are exceeded only by their independent interest in the classification of group actions on von Neumann algebras v TABLE OF CONTENTS 1 von Neumann Algebra Preliminaries and Constructions :::::::::: 1 1.1 Topologies on B(H), von Neumann algebras, and the Bicommutant Theorem 1 1.2 Type Classification of Factors . 5 1.3 States and the Gelfand-Naimark-Segal Construction . 8 1.4 Type II Factors . 11 1.4.1 Type II1 .................................. 11 1.5 Constructions . 13 1.5.1 Tensor Products of von Neumann Algebras . 13 1.5.2 Group von Neumann Algebras and Crossed Product Constructions . 15 1.6 Semifinite Algebras . 23 1.6.1 Weights and Traces . 23 1.6.2 Type II1 von Neumann algebras . 24 1.7 Crossed Products and Actions of Locally Compact Groups . 25 1.7.1 Cocycle Conjugacy . 28 1.7.2 The Modular Automorphism Group and Type III Factors . 30 2 Free Products and Voiculescu's Free Gaussian Functor ::::::::::: 36 2.1 The Full Fock Space, Free Product Construction, and Freeness . 36 2.2 Freeness with Amalgamation . 39 2.3 Free Creation/Annihilation Operators . 41 2.3.1 Dyck Paths . 41 2.3.2 Distribution of l(ξ) and l(ξ) + l∗(ξ)................... 43 2.4 The Free Gaussian Functor . 45 vi 2.5 Free Bogoliubov Actions . 47 2.5.1 Definitions and Outerness . 47 2.5.2 Mixing Representations and (Strong) Solidity . 48 3 Bimodules and Completely Positive Maps ::::::::::::::::::: 51 3.1 Complete Positivity and Stinespring Dilation . 51 3.2 Injectivity and Connes' Characterization of Amenability . 55 3.3 Bimodules . 57 3.3.1 Definitions and Examples . 58 3.3.2 Correspondence Between Bimodules and Completely Positive Maps . 64 3.3.3 A-valued Semicircular Families . 67 3.3.4 Intertwining by Bimodules . 72 4 Classification Results :::::::::::::::::::::::::::::::: 77 4.1 Technical Results and Preliminaries . 77 4.1.1 Rajchman Measures . 77 4.1.2 Corners Retain Spectral Data . 78 4.1.3 Embedding gives Corner Conjugacy of Actions . 80 4.2 The Main Result . 84 References ::::::::::::::::::::::::::::::::::::::::: 90 vii ACKNOWLEDGMENTS I'd like to first thank my advisor, Dima Shlyakhtenko, for his constant support. Without his valuable ideas, helpful comments, and unending patience, this work would not have been possible. I'd additionally like to thank Sorin Popa for offering useful thoughts, courses, and references throughout my time at UCLA. I'd also like to thank my parents and sister Kenzie for all of their love and encouragment. Our phone calls were a joy in times of stress. My friends have helped me in more ways than I can count, but I would like to specifically thank Josh Aguas, Ian Charlesworth, Ted Dokos, Jared Franco, Dustan Levenstein, Morgan Hallford, and Mikes Menke and Miller for all of their support throughout grad school. Finally, I'm indebted to UCLA for fostering a wonderful academic community of profes- sors and graduate students. I am very fortunate to have been a part of it. viii VITA 2013 B.S. in Mathematics Texas A&M University College Station, Texas ix CHAPTER 1 von Neumann Algebra Preliminaries and Constructions 1.1 Topologies on B(H), von Neumann algebras, and the Bicom- mutant Theorem Let H be a complex Hilbert space. We will usually be interested in the case where H is separable and infinite dimensional, in which case H has a countable orthonormal basis. For T : H ! H a linear operator, we define the operator norm of T to be: kT k1 = sup kT ξk: fξ2H:kξk=1g We say that a linear operator T : H ! H is bounded if the supremum in the above definition is finite and denote by B(H) the set of bounded linear operators on H. One can check that B(H) is precisely the set of continuous linear operators T : H ! H when H is equipped with the norm induced by its inner product. Note that B(H) forms an algebra over C whose multiplication operation is given by composition of operators: TS := T ◦ S. We note that the operator norm satisfies kTSk ≤ kT kkSk for all T;S 2 B(H). (Remark: We will suppress the 1 from the notation for the operator norm if there's no chance of confusion.) The algebra B(H) comes equipped with a natural conjugate-linear involution ∗ given by the adjoint. Recall that the adjoint T ∗ of an operator T 2 B(H) is the unique bounded linear operator satisfying for all ξ; η 2 H: hξ; T ηi = hT ∗ξ; ηi: We remark that the adjoint is an anti-homomorphism of rings (i.e. (TS)∗ = S∗T ∗) and that the operator norm satisfies kT k2 = kT ∗k2 = kT ∗T k for all T 2 B(H). 1 There are several natural topologies we can consider on B(H) which are weaker than the operator norm topology. For all of the following topologies and for Λ a directed set, we have that a net fTλ 2 B(H): λ 2 Λg converges to an operator T if and only if the net fTλ − T : λ 2 Λg converges to zero. So it suffices to describe which nets of operators go to zero. Definition 1.1.1 (Strong Operator Topology) We have Tλ ! 0 in the strong operator topology if and only if for all ξ 2 H, kTλξk ! 0: In other words, the strong operator topology corresponds to pointwise convergence in norm. If a set is closed in the strong operator topology, we will sometimes call it strongly closed. Definition 1.1.2 (Weak Operator Topology) We have Tλ ! 0 in the weak operator topol- ogy if and only if for all ξ; η 2 H, hTλξ; ηi ! 0: A closed set in this topology will sometimes be called weakly closed when there is no chance of confusion. Definition 1.1.3 (Ultraweak Topology) We have Tλ ! 0 in the ultraweak (or σ-weak) 2 topology if and only if for all (ξn)n2N; (ηn)n2N 2 l (N) ⊗ H, X hTλξn; ηni ! 0: n2N We note that the ultraweak topology is also the weak-* topology induced by the Banach space predual of B(H), which is usually represented as the set of trace-class operators on B(H). See [Hou11] for a more operator topologies on B(H).
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